In existing telecommunications networks, one protocol used for communication is known as synchronous transport mode (STM). According to this protocol, a given user of a communication path is allocated time slices at predetermined periodic intervals. A second common communication protocol is known as asynchronous transport mode (ATM). According to this protocol, a given user of a communication path receives time slices which are not periodic, but instead occur at variable or random points in time.
Faults occasionally develop in a telecommunications network, for example when an underground cable is broken by construction equipment. Over time, various protection techniques have been developed to facilitate automatic rerouting of data so as to ensure reliable and substantially uninterrupted service.
One such technique is known as one-plus-one (1+1) line protection. According to this technique, each operational or working cable is associated with a corresponding protection or back-up cable. The working cable and protection cable both carry the same operational data streams, but the data streams on the protection cable are ignored until a fault is detected in association with the working cable. More specifically, when a fault is detected in association with the working cable, the system implements appropriate switching so as to stop using the data streams received through the working cable, and begin using the data streams received through the associated protection cable.
A second protection technique is known as bidirectional line-switched ring (BLSR) protection. In this approach, half of the communication paths in each cable are used for high priority data, and the other half are used for low priority data. If a fault is detected in association with the first cable, transmission of low priority data on both cables is terminated, and then the high priority information from the first cable is switched over to the low priority communication paths in the second cable, after which all of the communication paths in the second cable will be carrying high priority data until the fault can be repaired.
A third protection technique is known as unidirectional path-switched ring (UPSR) protection. According to this technique, the detection of a fault does not necessarily result in the switching of data streams from an entire group of communication paths in one cable to a corresponding group of communication paths in a different cable. Instead, protection switching is carried out on a one-to-one basis among communication paths.
Traditionally, entirely separate switching circuits have been provided for STM and ATM communications. The switching circuits for STM communications usually include use of a protection technique, such as one of the techniques described above. In contrast, switching circuits for ATM communications typically include little or no protection against interruption of service due to a fault. This is due in part to the fact that the amount of STM traffic was once much greater than the amount of ATM traffic. In recent years, however, due to the popularity of personal computers and the Internet, the amount of ATM traffic through telecommunication networks has been progressively increasing at a rapid rate. As one consequence, there has been a progressively increasing demand from network users for ATM traffic to enjoy the same level of protection and reliability that has traditionally been enjoyed by STM traffic.
The use of separate switching circuits for STM and ATM traffic has also presented some other disadvantages. Separate parts must be inventoried for each type of system, and maintenance personnel must be trained on each type of system, which involves additional expense. Further, in order to change the ratio of STM communication paths to ATM communication paths, it is typically necessary to manually and physically disconnect a cable from an STM or ATM switching circuit, and then physically couple it to the other type of switching circuit. This must occur at both ends of the cable, which are typically in remote locations. Further, since it is a manual operation, it is extremely slow in relation to the speed of automatic operation and switching of the system itself. Another disadvantage is that, since there is virtually no common switching circuitry shared by the ATM and STM networks, and since reserve circuitry is needed in each such network in order to permit variation of the ratio of STM to ATM communication paths, excess circuitry must be provided in each network, which also increases the cost of maintaining a telecommunications network that meets user needs.